Condensed tannins (CTs) are polyphenols produced in the foliage and woody tissues of many plants. Condensed tannin concentration and molecular structure vary substantially within a species, and concentration also varies with leaf age. Differences in the concentration and molecular structure of CTs are associated with changes in interactions between CT-producing plants, their herbivores and the environment. Little is known about how CT molecular structures and associated chemical properties vary with leaf age, and by interaction with plant genotype. We investigated concurrent changes in the concentration and molecular composition of foliar CTs in three trembling aspen (Populus tremuloides) clones during the progression of a growing season, from vernal budbreak until autumn leaf senescence. Foliage was collected from five ramets of each aspen clone, and assayed for extractable and bound CT levels. Polymer chain lengths, relative subunit abundances, and stereochemistry were determined using thiolysis-ultra high performance liquid chromatography-mass spectrometry. Condensed tannin quantity and quality were compared over time and among clones using repeated measures analyses of variance (RM-ANOVA).
Results/Conclusions
Condensed tannin quantity and quality varied substantially across the growing season, and among P. tremuloides clones. In all three genotypes, foliar concentrations of extractable CTs were low at budbreak (<1% d.m.) in late April, increased rapidly during leaf expansion, remained relatively stable (6-10% d.m.) between June and August, and declined slightly toward autumn senescence (4-6% d.m.) in October. In contrast, concentrations of bound CTs were relatively stable and low (<2% d.m. on average) throughout the growing season. Condensed tannin quality also changed substantially during leaf expansion, with effect magnitudes and sometimes directions varying among genotypes. In general, between budbreak and June, CT polymer size and representation of the more biologically-active gallocatechin subunits approximately doubled, and was accompanied by substantial changes in molecular conformation. The quantitative and qualitative changes of CTs during leaf expansion were generally larger than variations among genotypes. Known relationships between CT concentrations, molecular structures, and biological effects suggest that the quantitative and qualitative changes that CTs undergo as leaves age during the course of a growing season are ecologically important. Moreover, factors influencing the temporal dynamics of foliar CT production have the potential to cause differential feedbacks to plant fitness, independently and by interaction with genotype.